Exposure apparatuses are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that retains a reticle, a lens assembly, a wafer stage assembly that retains a semiconductor wafer, and a measurement system. The reticle stage assembly and the wafer stage assembly are supported above a ground with an apparatus frame.
Recently, in order to increase the throughput of the exposure apparatus, wafer stage assemblies have been developed that include two wafer tables. In this design, each of the wafer tables retains a wafer. Further, a wafer mover assembly independently and alternately moves each wafer table with its wafer between an alignment position and an operational position. In the alignment position, the wafer is loaded onto the wafer table and the position of the chips on the wafer relative to the wafer table is determined. In the operational position, the images from the reticle are transferred to the wafer.
The size of the images transferred onto the wafer from the reticle is extremely small. Accordingly, the precise relative positioning of the wafer and the reticle is critical to the manufacturing of high density, semiconductor wafers.
In order to obtain precise relative positioning, the reticle and the wafer are constantly monitored by the measurement system. Stated another way, the measurement system monitors movement of each wafer table relative to the lens assembly or some other reference. With this information, the wafer mover assembly can be used to precisely position the wafer tables.
The measurement system typically includes one or more interferometers for monitoring the position of each wafer table. Typically, in the wafer stage assemblies that include only one wafer table, a first X interferometer system and a Y interferometer system are used to monitor the position of the wafer table in both the alignment position and the operational position. However, for wafer stage assemblies that include two wafer tables, a second X interferometer system is often necessary. In this design, the first X interferometer monitors the position of the wafer table along an X axis when the wafer table and the wafer is in the alignment position and the second X interferometer monitors the position of the wafer table along the X axis when the wafer table and the wafer is in the operational position. Further, the Y interferometer monitors the position of the wafer table and the wafer along a Y axis and about a Z axis when the wafer table is in both the alignment position and the operational position.
For each device table, the first X interferometer includes a first X mirror, the second X interferometer includes a second X mirror, and the Y interferometer includes a Y mirror. Each of the mirrors is secured to the device table. Unfortunately, the first X mirror and the second X mirror are not perfectly parallel. As a result thereof, the relationship between measurements taken with the first X interferometer and measurements taken with the second X interferometer is not precisely known. This reduces the accuracy of positioning of the wafer relative to the reticle and degrades the accuracy of the exposure apparatus.
In light of the above, one object of the present invention is to provide a stage assembly that precisely positions a device. Another object is to provide a method and system for determining the exact relative positions of the first X mirror and the second X mirror on each wafer table. Yet another object is to provide an exposure apparatus capable of manufacturing precision devices such as high density, semiconductor wafers.